In recent years, there are provided lateral-electric-field mode liquid crystal display devices, which is represented by an IPS (In-plane Switching) liquid crystal display devices, on the market. Such liquid crystal display devices have excellent viewing angle and are widely used in many information terminals ranging from comparatively large-sized devices, such as a television set and a monitor, to portable devices such as a tablet and a smart phone. An IPS liquid crystal display device includes two kinds of strip-shaped (oblong-shaped) electrodes (strip-shaped pixel electrodes and strip-shaped portions of a common electrode) both arranged in parallel at intervals, when viewed from a position perpendicularly above the substrates, and drives liquid crystal molecules by using an electric field applied between each set of the strip-shaped pixel electrode and the strip-shaped portion of the common electrode. Such IPS liquid crystal display devices are characterized by a feature that there is no major change in the view even when the direction viewed from the observer has changed. This is because the observer does not perceive a great change of the orientation of liquid crystal molecules in such display devices even after the viewing angle has been changed.
Accordingly, IPS liquid crystal display devices are characterized by a great viewing angle. In addition, some of IPS liquid crystal display devices may employ a multi-domain structure as follows, in order to enhance their viewing angle characteristics. That is, such IPS liquid crystal display devices may employ a structure where an electrode in each pixel is bent by arranging a part and the other part of the two kinds of strip-shaped electrodes in the following manners and joining the both parts together. That is, a part of the two kinds of strip-shaped electrodes is arranged so as to produce an electric field which is to be applied to the liquid crystal and forms +θ° angle between the direction of the electric field and the initial orientation of liquid crystal molecules. The other part of the two kinds of strip-shaped electrodes is arranged so as to produce the electric field which is to be applied to the liquid crystal and forms −θ° angle between the direction of the electric field and the initial orientation of liquid crystal molecules. It should be noted that the strip-shaped electrodes (strip-shaped pixel electrodes and strip-shaped portions of the common electrodes) do not need to have independent bodies. As far as edge portions of a pair of the opposing electrodes are in parallel, the end portion of each electrode may have any shape other than a strip shape, such that the end portions may be connected together, or a portion except the portion where the pair of electrodes are in the opposing positions may have a different shape.
However, in this multi-domain structure, each pixel includes a bent part at the junction of the electrode portion for the +θ° angle and another electrode portion for the −θ° angle. The bent part forms a domain boundary between a liquid crystal domain where there is a clockwise deformation and the liquid crystal domain where there is a counterclockwise deformation, which causes disclination of the liquid crystal molecules and results in a reduction of the transmittance.
A liquid crystal display device includes a plurality of scanning lines and a plurality of data lines both arranged on one of a pair of substrates, and a plurality of pixels. Each of the pixels is formed by a divided area whose boundary is defined by each two scanning lines and each two data lines out of the scanning lines and the data lines. The smaller the size of each pixel is, the larger the ratio of the area of scanning lines and data lines in each pixel, and as a result, the aperture ratio decreases. FIG. 20 is a graph showing a relationship between an aperture ratio and a pixel size wherein each of the scanning line width and the data line width is set at a constant value of 10 μm and a region of a pixel except the wiring area is assumed as an aperture. As can be understood from this relationship, the aperture ratio decreases prominently especially when the pixel size decreases to 100 μm or less. Accordingly, in liquid crystal display devices employing the multi-domain structure described above, there arises a problem that a decrease in the relative transmittance due to the bent part of an electrode in each pixel becomes too significant to be disregarded.
In order to avoid this problem, it is advantageous for the liquid crystal display devices to adopt a structure of a single domain where an electrode in each pixel is not bent. However, a use of the single domain structure results in loss of the outstanding viewing angle characteristics obtained with a multi-domain structure. To solve this problem, structures in which liquid crystal molecules rotate in two directions within each pixel without using a bent electrode, are described in Japanese Unexamined Patent Application Publications (JP-A) No. H09-105908 (FIGS. 32 and 36) and JP-A No. 2004-271971 (FIGS. 45, 46 and 48).
As a representative example of the conventional techniques, FIGS. 21 A and 21 B illustrate pixel structures disclosed in JP-A No. JP H09-105908 (in FIGS. 32 and 36), respectively, where upper and lower sides of the each figure are reversed in comparison with the original figures in order to simplify the following description. Each of FIGS. 21 A and 21 B shows common electrode 101, pixel electrode 102, electric fields 103 and 104, common electrode line 105, data line 106, source electrode 107, drain electrode 108, amorphous silicon (TFT channel region) 109, scanning line (gate line) 110, and boundary 111 of a black matrix. In the above structures, each or one of the common electrode 101 and the pixel electrode 102 additionally has an edge portion with an inclining edge such that one electrode gradually approaches to the other electrode as getting closer to the pixel end. In the approached region, the electrodes produce electric fields 103, 104 in two different directions. The document describes that such a structure makes two directions of rotation of liquid crystal molecules within a pixel, which can realize a multi-domain structure without using a bent electrode in each pixel. However, since each or one of the common electrode 101 and the pixel electrode 102 additionally has the edge portion with an inclining edge, the opening decreases in comparison with an area which could be used as the opening in the original structure, and it causes a decrease of the transmittance.
JP-A No. 2004-271971 also discloses in FIGS. 44 and 47 the above-described pixel structure that liquid crystal molecules rotates in two different directions within each pixel without using bent electrodes. As a representative example of the conventional techniques, FIG. 22 shows a pixel structure disclosed in JP-A No. 2004-271971 (in FIG. 44), where some components that are not used in the following description, such as accumulated capacitance lines and others, are omitted. FIG. 20 shows common electrode 101, pixel electrode 102, electric fields 103 and 104, data line 106, scanning line (gate line) 110, semiconductor layer 112, and through holes 113. Also in the disclosed structure, one electrode gradually approaches the other electrode, and the approaching region produces the electric fields 103 and 104 in two different directions. The document describes that such a structure makes two directions of rotation of liquid crystal molecules within a pixel, which can realize a multi-domain structure without using a bent electrode in each pixel. However, this structure also causes a decrease of the transmittance for the same reason as the reason described above.
Furthermore, these structures can cause the decrease of the transmittance for another reason, and the reason will be described with reference to FIGS. 23 and 24. FIG. 23 is an enlarged view of a region surrounded with broken lines in FIG. 22. In this structure, the common electrode 101 and the pixel electrode 102 are formed in different layers and there are some regions where the common electrode 101 and the pixel electrode 102 partly overlaps with each other within a pixel. In the overlapping regions and in specific neighboring regions, electric fields 114 and 115 in unwanted directions, which are different from those of the intended electric fields 103 and 104, are produced. These electric fields 114 and 115 are referred to as so-called fringe electric fields. Such a structure causes a rotation of liquid crystal molecules, which exist in an area affected by the electric fields 114 and 115 , in a direction opposite to the intended direction, which produces a reverse rotation domain 117. On the boundary of the reverse rotation domain 117 and the forward rotation domain 118, there are produced disclination 116, causing a decrease of the transmittance.
FIG. 24 illustrates situations of the disclination 116, the reverse rotation domain 117 and the forward rotation domain 118. Under the situation that a reverse rotation domain exists in proximity to the forward rotation domain, an external force, such as finger pressing, which has been temporarily applied onto the display screen during the drive of the display screen, makes a disturbance of the orientation of the liquid crystal molecules during the application of the force. When the screen is released from the external force, the liquid crystal molecules try to return to the original orientation again. However, it more likely causes a phenomena that the forward rotation domain is reduced as compared to a state before the external force is applied, and reversely, the reverse rotation domain becomes larger by an amount corresponding to the reduction and stabilizes in such a state. It causes a difference of the display condition between a display state before an external force is applied and a display state after an external force of finger pressing. As a result, there occurs a display issue recognized as finger pressing marks. This phenomenon causes a remarkable deterioration of the liquid crystal display device in the image quality.
The present invention seeks to solve the problems.